Electromagnetic actuators comprising an actuating member associated with an armature that can move between two extreme positions under the action of two electromagnets each comprising a coil and a core designed to channel a flux of the coil, so that it forms a return path in the armature, are known.
Such an actuator is for example used to actuate an internal combustion engine valve, the actuator being placed in such a way that the pushrod extends along the slide axis of the valve. The actuators are energized so that the armature is attracted selectively. The end of the pushrod and the end of the valve are brought back against one another by opposing springs that define an equilibrium position of the pushrod/valve assembly substantially at mid-path between the two electromagnets. The extreme positions of the armature are defined by the armature butting against the electromagnets and correspond to a closed position and to an open position of the valve respectively.
Each of the electromagnets includes permanent magnets that are incorporated into the core in such a way that the latter channels a flux of the permanent magnets so that it forms a return path in the armature. The permanent magnets serve to retain the armature in abutment on the corresponding core against the springs when the corresponding coil is not energized.
However, in case of a failure of the coil of the electromagnet returning the armature to the extreme position corresponding to the open position of the valve while the armature is kept against this electromagnet by the associated permanent magnets, it is then impossible for the armature to separate from the core and therefore to dislodge the valve from the open position.
The locking of a valve in the open position is a problem since, when the other valve of the same cylinder is opened, the valve short-circuits the intake circuit and the exhaust circuit of the engine, thereby preventing the engine from operating.
On the other hand, the locking of a valve in the closed position does not cause such a short circuit and the engine can continue to operate on the other cylinders.
It has been considered to use, for the electromagnet corresponding to the open position, an electromagnet having no permanent magnets, so as to prevent any locking of the armature in the extreme position corresponding to the open position of the valve. However, such an arrangement results in an unbalanced actuator, which is difficult to regulate.
This is because, for that one of the electromagnets that does not include permanent magnets, the retention of the armature against the corresponding core takes place without the coil being energized.
In contrast, for that one of the electromagnets that does not include permanent magnets, the retention of the armature against the core requires the coil to be energized so that it generates a flux equivalent to the magnets of the other electromagnet, that is to say a large flux.
The dissymmetry in the flux lines during retentions gives rise to regulating difficulties.
It has also been considered to offset the elastic member in such a way that the latter exerts a larger force when the valve is in the open position than when the valve is in the closed position. However, this offset gives rise to severe shocks when the armature butts on one of the electromagnets, which situation is unacceptable.